1. Field of the Invention
The present invention relates to a wire bonding apparatus and more particularly to a high-speed wire bonding apparatus that alleviates the impact on the object of bonding.
2. Prior Art
Wire bonding techniques connect, with a thin metal wire, input-output terminals, etc. (bonding pads) of semiconductor chips such as LSIs, etc. and terminals (bonding leads) on packages or circuit boards on which these semiconductor chips are mounted. Wire bonding apparatuses are those used in such techniques.
The wire is held by being passed through a tubular body called a capillary. Accordingly, the wire bonding apparatus has a mechanism that is used for the relative positioning of the capillary and the semiconductor chip or circuit board. For example, in a typical ultrasonic type wire bonding apparatus, the capillary is disposed on the tip end of a long slender rod-shaped ultrasonic transducer which is formed in a shape that is suitable for the transmission of ultrasonic energy, and the apparatus has a raising-and-lowering mechanism that moves the ultrasonic transducer so that the capillary is raised or lowered with respect to the semiconductor chip, etc.
For example, a mechanism which holds the ultrasonic transducer (with the capillary disposed on the tip end) by means of a holder, and which moves this holder by means of a linear motor, is used as the above-described raising-and-lowering mechanism. The overall system of the raising-and-lowering mechanism is controlled by a servo technique, and contact with the bonding pad is accomplished by controlling the displacement, speed and acceleration, etc. of the tip end of the capillary. Then, a specified pressing force is applied to the capillary, and bonding is next accomplished by applying ultrasonic energy to the wire and bonding pad.
Speed at which the capillary is lowered with respect to the bonding pad, etc. is one important factor in determining the time of the wire bonding operation. It is preferable that this lowering speed be as high as possible. However, if the capillary is lowered at a high speed and strikes the bonding pad “as is”, the IC chip, etc. will be damaged by the resulting impact. This impact depends on the magnitude of the inertia of the overall moving mechanism that lowers the capillary, and is a considerable impact in the case of current moving mechanisms that are equipped with an ultrasonic transducer, holder and raising-and-lowering mechanism, etc. Accordingly, a technique is used in which the lowering speed of the capillary is varied in stages, and the lowering speed is slowed after the tip end of the capillary has approached the bonding pad to within a certain proximity. In regard to the position at which the lowering speed is varied, a method is used in which variation in the thickness of the semiconductor chip or circuit board, etc. is taken into account; and, for example, the speed is reduced beginning at a position where the capillary has been lowered to a point that incorporates a safety margin in the amount of variation of the thickness.
Thus, in conventional techniques, control of the raising-and-lowering mechanism is made with the high-speed characteristics of the lowering speed of the capillary and alleviation of the impact taken into account.
In recent years, the size of LSI chips has increased and the number of bonding positions has increased; thus, there has been a need to increase the speed of wire bonding, and there has been a demand for an increase in the lowering speed of the capillary.
Furthermore, apart from the problem of damage to IC chips, there has been a demand for a substantial alleviation of the impact that acts on the object of bonding during the lowering action of the capillary as the ultra-miniaturization of electronic parts has progressed. In conventional techniques, wire bonding is performed on circuit boards, etc., whose lower portions are firmly supported by a carrying table; accordingly, there is no thought of these circuit boards, etc., flexing as a result of the impact that is applied during the lowering action of the capillary.
Recently, however, three-dimensional circuit assemblies have been designed for the purpose of ultra-miniaturization of electronic parts; and in such structures, there may be a need to perform wire bonding on a board that protrudes in the form of eaves, in which firm support cannot be obtained in the lower portion. In this case, since an impact is applied to the eave-form board when the capillary is lowered, wire bonding may become difficult as a result of problems such as flexing of the board and slipping of the capillary over the bonding pad, etc. Accordingly, there is a demand for a so-called impact-free wire bonding apparatus in which there is almost no impact during the lowering action of the capillary.
However, in conventional techniques, as a result of an increase in the lowering speed of the capillary and of an increase in the magnitude of the resulting impact, a method has been used in which the lowering speed is reduced after the tip end of the capillary has approached the bonding pad to within a certain proximity in order to alleviate the impact. Accordingly, if an attempt is made to alleviate the impact, then the speed of the capillary during lowering action is further reduced, hindering an increase in the speed of bonding.
As seen from the above, in conventional techniques, an increase in the speed of the capillary during lowering action and alleviation of the impact applied to the LSI chip by the capillary are conflicting requirements.
Furthermore, in conventional techniques, the mechanism that lowers the capillary has a complicated structure in which, for example, a capillary holding body such as an ultrasonic transducer is held by a holder, etc., and this holder is driven upward and downward. Accordingly, the inertia of the mechanism as a whole is considerable, and quick alteration of the lowering speed is difficult, so that there are limits to the extent to which the impact acting on the bonding pad can be alleviated.